LLM Training - How It Works

When you send a message to an LLM like ChatGPT or Claude, here's what happens at a high level:

A vector is just a list of numbers. That's it. The list `[0.2, -1.5, 0.8]` is a three-dimensional vector. The word "dimensional" sounds intimidating, but all it means is how many numbers are in the...

A token is the unit of text that an LLM actually works with. It's not a word, not a character — it's somewhere in between. The model has a fixed vocabulary of tokens (typically 30,000–100,000 of...

After tokenization gives you a sequence of token IDs like `[40, 3021, 5765, 18510, 540]`, the model needs to convert each ID into a vector the neural network can work with. This is the embedding...

When you send a message to an LLM, inference happens in two distinct phases. Understanding these two phases is key to understanding why LLMs perform the way they do.

There is no separate "thinking" module. The model doesn't have an internal scratchpad or a different mode of processing. Thinking is just **more token generation** — the model produces tokens that...

Tool calling is not a special capability baked into the model architecture. The model doesn't "run code" or "access the internet." It generates text that happens to follow a specific structured...

LLMs have no persistent memory. Every conversation starts from zero — the model has its weights (fixed, trained knowledge) and whatever tokens are in the current context window. When the context is...

Let's walk through what happens when you type "I love unbelievable tacos" and hit send.

A weight is a single number that the model can adjust to change its behavior. That's it. When people say a model has "405 billion parameters," they mean it has 405 billion individual numbers —...

Training works by repeated trial and error, automated at massive scale. Here's the loop:

This is one of the most surprising things about embeddings. Take the classic example: king - man + woman ≈ queen. What's actually happening in the vector math?

A layer is one complete round of transformation that every token vector passes through. If the model has 80 layers (typical for a large LLM), then each token vector gets transformed 80 times in...

The actual operations are matrix multiplications. Every transformation in a layer — attention, feed-forward — boils down to multiplying a vector by a weight matrix to produce a new vector.

Hidden states are simply what the token vectors are called once they're inside the model. Before layer 1, they're called embeddings. After passing through one or more layers, they're called hidden...

Take two vectors of the same length. Multiply their elements in pairs, then add up the results. That's the entire operation:

During prefill, every token at every layer produces three vectors: a **query** (Q), a **key** (K), and a **value** (V). The query is ephemeral — it's used once to compute attention scores against all...

A skill is not something the model does — it's something the **harness** (the software wrapping the model) does before and after the model runs.

Planning is an extension of thinking combined with tool use. The model doesn't have a planner module — it generates a plan as text, then executes it step by step through the normal token generation loop.

Tokenization runs on the **CPU**. Not the GPU. This surprises people because everything else in the LLM pipeline is GPU-bound, but tokenization is the wrong shape for GPU execution.

The embedding dimension (`d_model`) is a design choice made before training. Common values:

In practice, the model doesn't run attention once per layer — it runs it **multiple times in parallel** (typically 32-128 times), each with its own independent set of W_Q, W_K, W_V weight matrices....

After attention has mixed information between tokens, each token's vector passes through the **feed-forward network (FFN)** — and this is the solo step. No cross-token interaction whatsoever. Every...

After 80 layers of attention and FFN, each token's vector has been transformed into a rich contextual representation. But the model needs to produce an actual token — a word (or subword) from its...

Full attention means every token attends to every other token — T² comparisons. Sparse attention says: most of those comparisons are wasted. Token 4,000 probably doesn't need to attend to token 12....

Where sparse attention *skips* certain token pairs, approximation methods try to compute *something like* full attention but with less math. The goal is the same — avoid the T² bottleneck — but the...

Quantization means storing numbers with fewer bits. Every weight, every cached K/V vector, every activation is a number that takes up space in memory. The question is: how precisely do you need to...

The problem paged attention solves isn't the size of the KV cache per se — it's **fragmentation** and **waste** when serving many concurrent requests.

When the KV cache exceeds GPU HBM capacity, you have two choices: refuse the request, or move some of the cache to slower memory. Offloading chooses the second option — it creates a **memory...

The problem: in standard Multi-Head Attention (MHA), each of the 64 heads has its own K and V projections, so the KV cache stores 64 sets of K and V per layer. Most of the KV cache cost comes from...

**How multi-head splits the work.** Say `d_model` = 8,192 and you have 64 attention heads. Instead of running one attention operation across all 8,192 dimensions, you split the vector into 64 chunks...

The model doesn't "decide" it has fully answered your question. It has no concept of completeness, correctness, or satisfaction. It stops because it predicts a special token.